A substantial variety of designs and structures for heat-insulating containers have been proposed, and used commercially, for holding hot liquids such as beverages, as well as for holding hot foods. Although focused primarily on hot foods, containers of this invention are correspondingly suitable for holding cold liquids and cold foods. The primary problem addressed by the inventors herein is that known low cost disposable containers for holding hot liquids or hot foods generally either do not provide adequate thermal insulation, or do not provide adequate receptivity for decoration or other graphics representations on the outer surface of the container, or sheet stock structures are not amenable to facile fabrication into conical or cylindrically-shaped containers. Thus, conical-shaped paper cups are readily printed as sheet stock before being fabricated into cups and are well suited for containing hot liquids, but do not provide the insulation necessary for a user to be able to comfortably hold such container when the container holds hot (e.g. boiling temperature) liquid.
An alternative well known structure for low cost containers is expanded bead polystyrene foam cups, which provide excellent thermal insulation properties, but are generally physically weaker than paper cups, and do not well accept graphic design by printing.
Such expanded polystyrene foam container is e.g. prepared by casting unfoamed but foamable beads of polystyrene into a mold, and heating the polystyrene beads in the mold to develop the latent foam characteristics of the polystyrene beads, thus expanding the beads to form a foam cup in the mold. The so-fabricated container is then removed from the mold.
Alternatively, it is known to form a foamed styrene sheet, and to heat and thereby soften the so-formed sheet, and to heat-form the so softened sheet into container molds, to thereby form foamed cups therefrom.
In addition to the above disadvantages, such expanded polystyrene containers use precious petroleum resources and/or impose substantial volumetric load on waste streams such as incineration, land fill, and the like. As a further problem, a slow, inefficient and high waste printing process is required where it is desired to provide graphic representations on the outer surfaces of polystyrene foam heat-insulating containers, since printing can only be effected on already-fabricated containers.
Further, the tapered surface of such container contributes to blurring of any printing which is done at positions near the top and bottom of the container unless specialized and expensive printing technology is employed.
Another known conventional product is a double-wall heat-insulating paper container. Such containers cannot be manufactured at low cost because of the complexity of the manufacturing process. One example of such container is a container wherein an inner side wall layer of the body member is surrounded by a corrugated heat-insulating jacket. The process of manufacturing such container involves forming the corrugated jacket and bonding such corrugated jacket to the outer surface of an inner layer of a side wall of the container.
One disadvantage of such container, where the corrugated layer forms the outer surface of the container, is that conventional letters, figures, or other symbols so-resident on the corrugated outer surface are accordingly distorted during the process of forming the corrugations, and thus do not have aesthetic appeal to consumers. Another disadvantage is that such corrugated containers may not stack well and thus may require undesirably large volumes of storage space for warehousing, stocking, and the like.
Another example of heat-insulating containers has a dual cup-body structure wherein cooperating inner and outer cup bodies have different e.g. conical angles to a reference direction, so as to define a heat insulating chamber between the inner and outer cup bodies. The two cup bodies are joined together by curling upper portions of the respective cup bodies to form a single rim and thus to form the composite cup. Thus, the insulating chamber has a minimum cross-section adjacent the rim, and a larger cross-section adjacent the bottom of the cup, whereby the effect of the insulation properties is least adjacent the upper rim and greatest near the bottom of the cup, and generally graduated in value between the upper rim and the bottom of the cup.
The outer surface of the side wall of the outer cup body is generally sufficiently smooth to be receptive to high resolution printing as a sheet material prior to being fabricated into a cup. While graphics prospects are thus improved, the rims may easily separate, and such cups require substantial space for storage and warehousing, especially because the different conical angles preclude close nesting of an overlying cup inside an underlying cup. In addition, the manufacturing cost is undesirably high because of the multiple cup bodies in combination with the required space between the layers.
In yet another known container, the outer surface of the side wall is a heat-insulating layer of a relatively lower melting temperature foamed thermoplastic resin film, and the inner surface of the side wall is coated or laminated with a relatively higher melting temperature thermoplastic resin film. When manufacturing such container, the moisture in the intervening paper layer is vaporized by applying heat, and the vaporized moisture causes the relatively lower-melting temperature thermoplastic resin film, which is disposed outwardly of the paper layer, to expand as foam.
Such containers have an advantage of exhibiting fairly good heat insulating properties, and can be manufactured at relatively low cost by a relatively simple process. However, the moisture level in the paper layer must be controlled carefully, lest the relatively low melting temperature film not foam adequately when the paper layer is heated. Thus, while relatively higher water content is advantageous for the purpose of developing the foam in the film, the mechanical strength of a container so formed may be substantially less than optimum where such high levels of moisture are present in the paper layer. If, however, moisture level in the paper is too low, the low melting temperature film is not adequately foamed and the structure is defective for failure of thermal insulating properties.
Yet another container is known wherein a heat-insulating paper container includes a side wall comprising a paper layer wherein part but not all of the outer surface of the paper layer is printed with an ink having a volatile solvent. A thermoplastic resin film is subsequently coated over the printing layer. When the so-coated substrate is heated, the over-coated thermoplastic resin film forms a relatively thicker foamed heat-insulating layer in the printed area of the outer surface and a relatively less thick foamed heat-insulating layer in the non-printed area.
In fabricating such containers, the printing ink is thus disposed on the paper layer, and is then overcoated with the subsequently-foamed polymer layer, whereby the printing is obscured by the foamed layer which overlies the printed areas. Consequently, the quality of cup decoration, graphics, text messaging and the like are perceived as less than desirable.
Still another known container uses the polymer resin of a printing ink in an outer layer of the side wall to control, namely to restrict, expansion of an underlying foamable polymer layer to a degree of foaming which is less than the potential degree of foaming for that polymer. As a result, especially heavily printed areas of the outer surface layer expand less than unprinted areas.
As a further method of controlling the extent of foaming, indeed to counteract the restrictive control imparted by the printing ink, a mineral oil can be coated onto the foamable layer to pre-soften the foamable polymeric material, thereby to facilitate development of foam cells in the underlying polymeric thermoplastic layer.
Accordingly, there is a need for a low cost, highly insulating sheet material which can readily be fabricated at low cost into especially container side walls for use in fabricating cups and related especially conically-shaped containers.
Thus, it is an object of the invention to provide low cost substrate sheet material having an expanded foam layer, and which sheet material is readily utilized to form side walls of especially truncated conical-shaped containers, namely tapered cups.
It is another object of the invention to provide an insulating cup or other container having an expanded foam side wall having peaks and valleys in the remote surface of the expanded foam layer.
It is yet another object to provide such cup or other container wherein insulation is provided by dead air space, either between the expanded foam layer and an overlying cover layer, or between the expanded foam layer and the fingers a user uses to grip such container, or between the expanded foam layer and a thermoplastic heat seal layer.
It is still another object to provide novel processes for forming such container substrate material and for forming cups and other containers using such substrate material.
It is a still further object to provide methods for fabricating such substrate material wherein the foam layer is disposed between the paper layer and a thermoplastic heat seal layer, and wherein the heat seal layer is coated onto the foam layer with sufficient heat to thermally bond the film to the foam layer but at sufficiently low temperature that the foam layer is not deleteriously affected by such bonding heat.